A navigation system performs travel guidance for enabling a user to easily and quickly reach the selected destination. A typical example is a vehicle navigation system where a vehicle is equipped with a navigation function to guide a driver to a destination through a calculated route. Such a navigation system detects the position of the user's vehicle, and reads out map data pertaining to an area at the current vehicle position from a data storage medium, for example, a CD-ROM (compact disc read-only memory), a DVD (digital versatile disc), a hard disc, or from a remote data server.
FIGS. 1A-1H show a typical example of overall procedure and screen display involved in the navigation system. FIG. 1A shows an example of locator map screen of the navigation system when the destination is not specified. A process for specifying a destination typically starts through a main menu screen of FIG. 1B which displays menu items including a “Destination” menu for entering the destination. When selecting “Destination” in FIG. 1B, the navigation system displays a screen of FIG. 1C for specifying an input method for selecting the destination.
By selecting, for example, a “Point of Interest” method in FIG. 1C, the navigation system displays selection methods of point of interest (POI) either by “Place Name” or “Place Type” in FIG. 1D. If the “Place Type” is selected in FIG. 1D, the navigation system lists categories of POIs as shown in FIG. 1E through which the user selects a desired category of POIs from the lists.
FIG. 1F shows a screen when the user has selected a “Fast Foods” category in the example of FIG. 1E. Then, the screen lists names of the fast food restaurants typically sorted by distance from the current position of the user. If the user selects a particular fast food restaurant from the list, the navigation system calculates a route to the destination as shown in FIG. 1G.
After determining the route, the navigation system starts the route guidance operation. As shown in FIG. 1H, the navigation system shows the intersection for instructing the next turn and a direction of the turn. Typically, such an instruction to turn will be made by a highlighted arrow or large characters on the screen as well as by a voice announcement when the vehicle approaches to within a predetermined distance to the intersection.
However, a distance to the maneuver point of the intersection may not be appropriate with respect to a timing of announcing the maneuver action depending on circumstances of the intersection. For example, inaccuracy of such distance or timing is caused by a digitization method used in the map data where a maneuver point is defined as a crossing point of center lines. The problem associated with such a conventional method is described with reference to FIGS. 2A and 2B.
FIGS. 2A and 2B show a case where a vehicle 29 is to make a right turn at an intersection 20 where a crossing point 21 is a reference point for determining a distance to the maneuver point. In the case of FIG. 2A, an intersection 20a has a large size because each cross street has three lanes at each side. In the case of FIG. 2B, an intersection 20b has a small size because each cross street has only one lane at each side.
In the example of FIG. 2A, when the vehicle 29 approaches the intersection, an actual point for making a right turn is a corner point 23. However, since the timing of announcing the maneuver is determined based on the distance from the reference point 21, such announcement of “Make a Right Turn” may be made when the vehicle is already at or very close to the corner point 23. Such a maneuver instruction is too sudden and surprises the driver and adversely affects the safe driving.
More specifically, as shown in FIG. 2A, the crossing (reference) point 21 is established by two center lines of the streets that form the intersection 20a. The distance represented by “dr” in FIG. 2A is a reference distance between the crossing point 21 and the vehicle 29. In the conventional technology, the reference distance “dr” is predetermined such as 40 feet so that when a vehicle reached this distance range from a crossing point 21, the navigation system produces the maneuver instruction.
The conventional technology may work well if an intersection is small as in the case of the intersection 20b shown in FIG. 2B. However, if an intersection is large as in the case of FIG. 2A, the fixed reference distance “dr” causes the problem as noted above. As a result, the instruction to turn maneuver may come too late, which fails to leave enough time for the driver to make the maneuver securely. In the worst case, such an instruction for maneuver may not be produced even when the vehicle is already within the intersection, which may scare the driver and seriously impede the safe driving.
Therefore, there is a need of a new method and apparatus for navigation system to produce the maneuver instruction such as voice announcement at a more appropriate timing. There is need of a new method and apparatus for navigation system to produce the maneuver instruction at an appropriate timing even when the size and structure of the intersection, directions of turn, etc. vary.